Vacuum switch



Nov. 20, 1962 D. A. GUNTHER 3,064,323

VACUUM SWITCH Filed Sept. 13, 1961 2% 4 IN VEN TOR. FIG. I am United States Patent ()1 3,064,323 VACUUM SWITCH Donald A. Gunther, Erie, 1221., assignor to American Sterilizer Company, Erie, Pa., a corporation of Pennsylvania Filed Sept. 13, 1961, Ser. No. 137,843 2 (Iiairns. (Cl. 21--103) This invention is a vacuum switch for operation throughout the range from atmospheric pressure down to ten microns. The switch is based upon the variation in the boiling point of water in accordance with absolute pressure and includes means for causing the water to boil continuously and evenly so that the actual temperature of the water at any pressure corresponds to its theoretical value. One use of such switches is in sterilizers where a vacuum is drawn prior to the introduction of the sterilizing medium in order to insure thorough and complete load penetration. The continuous and even boiling of the water avoids super heat which could cause a lack of correspondence between the temperature of the water and the vacuum.

In the drawing, FlG. 1 is a diagrammatic view illustrating the operation of the switch, FIG. 2 is a diagrammatic view of a sterilizer equipped with the vacuum switch and FIG. 3 is a modification.

The principle is illustrated in FIG. 1 where 1 indicates a test tube having an air tight stopper or closure 2 at the top and filled with water to the level indicated at 3. Extending through the closure 2 is a thermometer 4, capillary tube 5, and a tube 6 connected to a vacuum pump. The thermometer and the capillary tube 5 extend close to the bottom of the test tube so that the lower ends are well below the surface of the water. The suction tube 6 extends only a short distance through the closure tube so that its lower end 7 is well above the surface of the water.

As the space above the water is evacuated, the absolute pressure decreases and the boiling point of the water is lowered. Theoretically, the boiling point corresponds exactly to the absolute pressure within the evacuated space. However, it has been observed that the boiling point of the water does not correspond exactly to the absolute pressure due to super heating. That is, the water does not boil smoothly and evenly but in intermittent bursts. This defect is overcome by causing the water to boil continuously and evenly. This is accomplished by the capillary tube 5 which has its outer end 8 connected to atmospheric pressure through a clamp 9. When the clamp is open, a continuous stream of air is discharged through the lower end 10 of the capillary tube adjacent the bottom of the test tube 1, causing a stream 11 of fine bubbles to pass up through the water. These fine bubbles insure continuous and even boiling of the water so that the temperature of the boiling water corresponds accurately to the absolute pressure within the evacuated space. The clamp 9 provides a control on the air admission. Adjustment of the clamp cuts down the air flow to that required to maintain the continuous stream of fine bubbles 11.

FIG. 2 shows the application of the vacuum measuring device of FIG. 1 to a steam sterilizer having a chamber 12 which is loaded with material to be sterilized and is then closed by a door 13. In order to secure uniform penetration of the load, a vacuum is drawn through line 14 under the control of valve 15. When the vacuum reaches the desired value, for example in the range of fifteen to twenty millimeters of mercury absolute, a control 16 starts the sterilizing cycle with the admission of steam through line 17 under the control of valve 18. The details of the sterilizing cycle are not important. It

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is important that the sterilizing cycle start only after the required vacuum has been reached in the sterilizing chamber. This is accomplished by a thermostatic switch 19 in the power circuit for the control having a thermostatic bulb 29 extending up through the lower end of a well 21 having its upper end connected by a l ne 22 to the upper part of the sterilizing chamber. During steam sterilizing, the well 21 is subject to steam pressure and condensate collects in the well to a suitable level indicated at 23. The well 21 is outside the sterilizing chamber in an ambient temperature well below the steam ternperature. A thermal insulating fitting 22a in line 22 1nsures a well temperature low enough for condensation. It is possible that the condensate may more than fill the well 21, but this will do no harm. The excess merely overflows into the sterilizing chamber. Accordingly, at the end of each sterilizing cycle, the well 21 is adequately filled with condensate. When the sterilizing chamber is evacuated at the start of the sterilizing cycle, the absolute pressure is decreased and when the pressure is low enough, the water in the well 21 boils. The boiling point of the water is a measure of the absolute pressure or vacuum within the sterilizing chamber.

To insure accurate correspondence between the temperature of the water in the well 21 and the vacuum in the sterilizing chamber, a capillary tube 23a is connected through a fitting 24 into the lower part of the well 21. The air discharged into the well by the capillary tube causes a stream 25 of fine bubbles to flow up through the water, causing continuous and even boiling so that the temperature of the boiling water corresponds accurately to the absolute pressure. The switch 19 controlled by the thermostat bulb 20 accordingly closes and initiates the control cycle at the desired vacuum.

Thermostats for the control are readily available. For example, if the desired vacuum were in the range of from ten to twenty millimeters, the thermostat would have an operating range of from slightly over 52 degrees to slightly less than 72 degrees. This could be a thermostat 0f the type used to control room temperature. Thermostats for other ranges of vacuum are readily available.

In the arrangement of FIG. 2, there is a continual bleed of atmospheric air through the capillary tube into the chamber being evacuated which is objectionable at high vacuums. To overcome this objection, the capillary tube 23a is connected to an expansion chamber 26. At the start of the evacuation of the sterilizing chamber 12, the .air in the expansion chamber 26 is at atmospheric pressure. The pressure in the expansion chamber 26 is the same as in the sterilizing chamber 12 because they are connected together by the line 22. Accordingly, during increasing vacuum (decreasing pressure) the air in the expansion chamber expands and bubbles out through the capillary 23a .and fitting 24 into the water in the bottom of the well 21. This prevents super heating of the water and sudden ebulition which causes both erratic operation of the thermostatic switch and loss of water through the vacuum line 22 due to violent boiling. The expansion chamber 26 eliminates the continuous leakage of outside air which is undesirable at high vacuums.

This application is an improvement on application Serial No. 91,714 filed February 27, 1961.

What is claimed as new is:

1. In a sterilizer having a sterilizing chamber with means for supplying steam to the chamber, means for evacuating the chamber prior to the admission of steam, a switch to be operated when the vacuum in the chamber reaches a predetermined value, a well outside the chamber, a line from the well to a part of the chamber above the well so that steam from the chamber is condensed by the cooling elfect of the ambient air and maintains the well filled with water, a thermostat immersed in the water for operating the switch as the boiling point of the water in the well reaches the temperature corresponding to said predetermined value of vacuum, the improvement which comprises a closed expansion tank, a capillary tube having one end connected to the tank and having the other end below the surface of the liquid in the Well, said tank being filled through the capillary tube with air from the sterilizing chamber upon opening of the sterilizing chamber, and the vacuum in the sterilizing chamber drawing air from said expansion tank through the capillary tube and discharging it below the surface of the liquid in the well in a fine stream of bubbles which promote smooth and even boiling of the liquid and preventing superheat.

2. In a vacuum switch having a well filled with liquid which boils at temperatures Within the ranges of vacuum at which operation of the switch is desired, a line connecting the well to the source of vacuum, and a thermostatic switch responsive to the temperature of the liquid in References Cited in the file of this patent UNITED STATES PATENTS 867,907 Bond et al. Oct. 8, 1907 2,635,456 Barstow Apr. 21, 1953 2,671,342 Kapfi Mar. 9, 1954 OTHER REFERENCES Perry: Chemical Engineers Handbook, 3rd ed.; McGraW-Hill Book Co., Inc., 1950'; p. 1176. 

